Joint evolution of the biogeography and phenology of seasonal migration.

In migratory species, the temporal phases of the annual cycle are linked to seasonally shifting geographic ranges. Despite intense interest in the annual cycle ecology of migratory species, a synthetic understanding of the relationship between the biogeography of the migratory annual cycle and its phenology remains elusive. Here, we investigate the spatiotemporal structure of the annual cycle in a phylogenetic comparative framework by developing a method to demarcate the pacing of annual cycle stages using eBird, a massive avian occurrence dataset, and applying it to migratory passerine birds breeding in North America. Our analyses reveal a striking negative correlation between the durations of the breeding versus nonbreeding periods, indicating that a tradeoff between the lengths of the two stationary periods is the primary axis of variation in annual cycle pacing. Our results further show that the duration of annual occupancy in the breeding versus stationary nonbreeding ranges predicts the geographic separation of these seasonal ranges, demonstrating that the ratio of time spent on stationary breeding versus nonbreeding locations evolves in tandem with a species' migration distance. In contrast, the amount of time during which species undergo seasonal migration-that is, the duration of the seasonal periods when species' geographic ranges shift latitudinally-varies relatively little across species compared with the length of the stationary periods. Our study helps untangle the complexity of seasonal distributions and schedules to reveal the integrated evolution of the biogeography of the migratory cycle, its pacing, and life history tradeoffs among species.